Could you be a little more specific, maybe with a score sample? I'm not even sure which Dittersdorf concerto you're referencing.
I assume you're talking about a double stop with F# and A in fourth position, but I don't see that anywhere in the first movement until the cadenza (Concerto No. 2 in D major). If that's what you mean, you can try using 1-3 instead by collapsing the last joint of your first finger. Some people might tell you to never collapse your joints, but those people are crazy.
And if it's a double stop, you should remember to focus your bow's contact onto the lower string and just barely touch the higher string. This will almost always get a much better tone.